1. Field of the Invention
The present invention relates to a signal processing circuit and, particularly, to a signal processing circuit and a signal processing method for removing noise due to multipath.
2. Description of Related Art
A receiving device in FM radio or the like performs noise reduction due to multipath. A receiving device usually receives a radio wave that is transmitted through a plurality of paths. The noise due to multipath is noise that is generated due to a difference in the time of receipt among a plurality of transmission paths. Japanese Unexamined Patent Publication Nos. 2001-285096 (Nakazawa) and 2002-271219 (Gamo) disclose a technique for reducing such noise.
Japanese Unexamined Patent Publication Nos. 2005-277565 (Saito et al.) also discloses a technique for reducing multipath noise. FIG. 11 is a block diagram showing the configuration of the circuit which is described in Saito et al. The circuit taught by Saito et al. includes a noise attenuation processing unit 140, an attenuation factor setting unit 142, a differential threshold setting unit 144, a modulation factor detection unit 146, a multipath detection filter 148, a level detection unit 150, an electric field strength detection unit 152, a filter coefficient setting unit 154, a multipath detection unit 156, an output control unit 158.
The noise attenuation processing unit 140 detects the period when multipath noise occurs, and it also attenuates an input stereo composite signal and outputs it to the output control unit 158.
The attenuation factor setting unit 142 sets an attenuation factor of a stereo composite signal according to an output of the modulation factor detection unit 146, an output of the level detection unit 150 and an output of the electric field strength detection unit 152, and outputs the attenuation factor to the noise attenuation processing unit 140. The attenuation factor setting unit 142 includes an attenuation factor table 162 to set an attenuation factor. By reference to the attenuation factor table 162, the attenuation factor setting unit 142 sets an attenuation factor according to the level of an input signal. The attenuation factor setting unit 142 further includes a time controller 160 that controls time based on a noise detection signal which is output from the noise attenuation processing unit 140.
The differential threshold setting unit 144 sets a threshold to serve as a reference to judge a difference according to an output of the modulation factor detection unit 146, an output of the level detection unit 150 and an output of the electric field strength detection unit 152, and outputs the threshold to the noise attenuation processing unit 140. The differential threshold setting unit 144 includes a threshold table 164. By reference to the threshold table 164, the differential threshold setting unit 144 sets a threshold.
The modulation factor detection unit 146 detects the amplitude of a stereo composite signal, which is the intensity of the stereo composite signal, and outputs the detected intensity to the attenuation factor setting unit 142 and the differential threshold setting unit 144.
The multipath detection filter 148 extracts the AC component around 19 kHz, for example, and outputs it. The multipath detection filter 148 adjusts the filter properties according to an output of the filter coefficient setting unit 154.
The level detection unit 150 outputs a signal that indicates a change in the envelope of a signal which is output from the multipath detection filter 148 to the attenuation factor setting unit 142 and the differential threshold setting unit 144. The electric field strength detection unit 152 detects electric field strength. The electric field strength detection unit 152 outputs a signal that indicates the electric field strength to the attenuation factor setting unit 142 and the differential threshold setting unit 144.
The filter coefficient setting unit 154 sets the filter properties of the multipath detection filter 148 according to an output of the electric field strength detection unit 152. The multipath detection unit 156 detects the generation of multipath noise based on an output of the level detection unit 150. The multipath detection unit 156 outputs a control signal that indicates the generation of multipath noise to the output control unit 158.
The output control unit 158 receives an output signal of the noise attenuation processing unit 140 and a stereo composite signal. The output control unit 158 sets an amplification factor of the output signal of the noise attenuation processing unit 140 and the stereo composite signal according to a control signal. Based on the amplification factor, the output control unit 158 adds the amplified output signal of the noise attenuation processing unit 140 and the amplified stereo composite signal together and outputs the result. The output control unit 158 includes an amplifier 166 with an amplification factor of “a” that sets the level of a stereo composite signal, an amplifier 168 with an amplification factor of “b” that sets the level of an output signal of the noise attenuation processing unit 140, and an adder 170 that adds an output of the amplifier 166 and an output of the amplifier 168 together and outputs the result.
The output control unit 158 increases the amplification factor b if the control signal indicates the detection of multipath noise, and it increases the amplification factor a if the control signal does not indicate the detection of multipath noise. FIG. 12 shows an example of the output control by the output control unit 158. The value of the amplification factor a of the amplifier 166 is designated by a solid line, and the value of the amplification factor b of the amplifier 168 is designated by a dotted line.
When t<t1, a=1 and b=0. In such a case, no multipath noise is detected, and the output control unit 158 outputs a stereo composite signal only.
When t1<t<t2, the amplification factor a decreases gradually and the amplification factor b increases accordingly because multipath noise is detected at the time t1. Thus, the output of the stereo composite signal from the output control unit 158 decreases as time elapses, and the output signal of the noise attenuation processing unit 140 increases as time elapses.
When t2<t, a=0, and b=1. Therefore, the output control unit 158 outputs the output signal of the noise attenuation processing unit 140 only.
The output control unit 158 performs the same control in the event of the transition from the state where multipath noise is detected to the state where multipath noise is no longer detected.
In this manner, upon changing the output, the output control unit 158 implements the fade-in fade-out control of the stereo composite signal and the output signal from the noise attenuation processing unit 140, thereby preventing the output from a signal processing circuit 136 to be discontinuous.
However, the technique described in Saito et al. switches between a demodulated signal and an output of the noise attenuation processing unit 140 based on the presence or absence of multipath which is detected by the multipath detection filter 148, the level detection unit 150 and the multipath detection unit 156. Specifically, if multipath exceeds a threshold, the signal processing circuit 136 selects and outputs the signal whose noise is attenuated by the noise attenuation processing unit 140 as shown in the region of t2<t in FIG. 12. On the other hand, if multipath falls below the threshold, the signal processing circuit 136 outputs only the stereo composite signal whose noise is not attenuated at all as shown in the region of t<t1 in FIG. 12. Accordingly, even if multipath occurs, the signal whose noise is not removed at all is output unless the multipath exceeds a threshold. Further, if the presence of multipath is detected, the signal processing circuit 136 always outputs the output signal from the noise attenuation processing unit 140 only, so that a signal with a fixed degree of attenuation is output regardless of the level of multipath.
The present inventor has recognized that the technique taught by Saito et al. outputs a signal without any noise attenuation or a signal with excessive noise attenuation with respect to an original demodulated signal for a long period of time, which can cause adverse effects such as degradation of stereo separation and production of muffled sound.
The present inventor has also recognized that the technique taught by Saito et al. can cause unnecessary loss of sound quality for the sake of multipath noise reduction.